Which types of closed questions are unlikely to be deleted?

Question

Because of the lack of any meaningful information on deletions, the only way for people who don't work for StackExchange to do useful statistical analysis on question closures (to answer questions like "Just how badly is the close queue backlog affecting close speed?") is live-scraping the real site using the StackExchange API. Otherwise, it's too biased. You end up with crazy results - like that the average time for closing a 5 year old Java question is about 5 years, and question closing gets monotonically faster and faster every month!

So are there any types of closed questions that are unlikely to be deleted?

What about closes as duplicates? Duplicates are useful, right? At least, we used to think so.

Answer 1

On Code Review, I tend to preserve closed questions that have useful (mostly upvoted) answers. Closure still tells others that this type of question is off-topic, but the answerers don't need to be penalized for helping the OP and giving useful contributions in scope with the question.

What about closes as duplicates? Duplicates are useful, right? At least, we used to think so.

According to this post, exact duplicates can be deleted. I assume the most likely case involves migrations, where the OP is directed to the target site while the post itself is migrated.

Other duplicates, as you've mentioned, shouldn't be deleted.